Plant and Soil 246: 41–52, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. 41 Phosphorus efﬁciency of wheat and sugar beet seedlings grown in soils with mainly calcium, or iron and aluminium phosphate Pratapbhanu S. Bhadoria 1 , Bernd Steingrobe 2,3 , Norbert Claassen 2 & Horst Liebersbach 2 1 Agricultural and Food Engineering Department, Indian Institute of Technology, Kharagpur-721302, India. 2 Institut für Agrikulturchemie, Georg-August-Universität Göttingen, Carl-Sprengel-Weg 1, D-37075 Göttingen, Germany. 3 Corresponding author ∗ Key words: external P requirement, internal P requirement, Luvisol, model, Oxisol, P inﬂux, root morphology Abstract Phosphorus is often limiting crop growth in soils low in P supplying capacity. The objective of this study was to investigate whether there are differences in P efﬁciency between sugar beet and wheat and to search for the plant properties responsible for different P efﬁciencies encountered and furthermore to see whether the kind of P binding in soil affects the P efﬁciency of crops. For this a pot experiment with an Oxisol with P mainly bound to Fe and Al (Fe/Al-P) and a Luvisol with P mainly bound to Ca (Ca-P) was run with increasing P fertilizer levels from 0 to 400 mg kg -1 in a climate chamber. Shoot dry weights of wheat and sugar beet increased strongly with P application in both soils. Both crops, despite their large differences in plant properties, had the same P efﬁciency in both soils. Therefore none of the species was especially able to use either Fe/Al-P or Ca-P. Wheat relied on a somewhat lower internal requirement, a large root system (high root/shoot ratio) and a low shoot growth rate with a low inﬂux while sugar beet with a small root system and a large shoot growth rate relied on a 5 to 10 times higher inﬂux. A mechanistic mathematical model for calculation of uptake and transport of nutrients in the rhizosphere was used to assess the inﬂuence of morphological and physiological root properties on P inﬂux. A comparison of calculated and measured P inﬂux showed that prediction by the model is reasonably accurate for Luvisol. For Oxisol, the predicted P inﬂux was much less than the observed one, even when P inﬂux by root hairs was considered. A sensitivity analysis showed that physiological uptake parameters like I max , K m , and C Lmin had no major inﬂuence on predicted inﬂux. The greatest inﬂuence on inﬂux had the P soil solution concentration C Li . It is assumed that both species had used mechanisms to increase P availability in the rhizosphere similar to an increase of C Li . Such mechanisms could be the exudation of organic acids, which are known as a sorption competitor to phosphate bound to Fe/Al-oxides or humic-Fe-(Al) complexes or to build soluble complexes with Fe and P. The close agreement between calculated and measured P inﬂux in the Luvisol even at P deﬁciency indicates that root exudates were not able to mobilize Ca-bound P, whereas Fe/Al-P could be mobilized easily. Introduction Phosphorus is often limiting crop growth particularly in low P soils of high Fe/Al or Ca contents, where P is strongly bound and largely unavailable for crop up- take. Plant species differ in their ability to grow under these conditions, i.e., they differ in their P efﬁciency. In this paper we deﬁne P efﬁciency by the relative yield of the plant at low as compared to sufﬁcient P supply. The differences in P efﬁciency may be very ∗ FAX No: +49-551-395570. E-mail: bsteing@gwdg.de large as was already shown by Lilleland et al. (1942) cited by Black (1968). They found on a P-deﬁcient soil that for example squash reached 3%, wheat 38%, alfalfa 57%, and almond 100% of the maximum yield achieved at high P supply. Differences in P efﬁciency can arise in three ways. (1) Efﬁciency with which P is utilized in the plant to produce ﬁnal yield. This is often called internal P re- quirement and is the P concentration in the plant to produce for example 90% of maximum yield. (2) The uptake efﬁciency is the ability of the root system to acquire P from soil and accumulate it in the shoots.